1. Field of Invention
The present invention relates to a method and means for drying various fibre materials which are used for making paper, board or the like and in particular for drying wood pulp.
When pulp is produced, water is removed from it by means of pressing and drying until its dryness becomes about 90%. Several different methods are known for drying the pulp, however all have certain disadvantages.
2. Description of the Prior Art
According to one widely used method the pulp is dried in the form of a web, after water has first been removed from it by means of press rolls, the web is conveyed around and pressed against drying cylinders so that the heat required for drying is transferred to the web from the hot metal surface of the cylinders.
Various methods where the drying heat is provided by blowing hot gases against the web are also known.
Common to both of the above mentioned methods, is that the initial phase of the drying is rather rapid, but when the water between the fibres has been removed and the dryness of the web has increased from 40-50% to 60-70%, the drying rate decreases considerably. Therefore in order to reach a dryness of 90% a bulky and expensive dryer is required, which causes high building costs. It is also difficult to defibrate the dried web because of the very strong fibre bondings resulting from the combination of the pressing and subsequent drying to 90%.
Pulp is also dried as flakes, whereby the wet web, from which water has been removed by pressing, is disintegrated and fed into a drying tower together with hot air from an air heater. The initial capital outlay and the space requirement of the flash dryer, as it is called, is smaller than the above mentioned systems, but the power requirement of the process is high. One other drawback of the method is the heterogenity of the dried pulp. When disintegrating the wet web in a shredder, highly compressed spots of fibre clusters result which have a lower drying rate and which are more difficult to defibrate than other parts of the flake.
Is is also well known in the prior art to dry a wet porous web by passing drying air through the web, but the air-through drying method can not be applied economically to webs having a basic weight in the heavier ranges of 500-1500 g/m.sup.2, from which water has been removed by pressing, because the web is not sufficiently porous to allow the drying air to pass through it.